This invention relates generally to incremental printing, with transversely scanning printheads, onto a printing medium that is also advanced along a longitudinal axisxe2x80x94and more particularly to marking-position errors in the printing-medium-advance axis. These errors are particularly important in that they create perceptible and in some cases conspicuous defects in printed images.
(a) The printer mechanismxe2x80x94The invention is amenable to implementation in a great variety of products. It can be embodied in a printer/plotter that includes a main case 1 (FIG. 1) with a window 2, and a left-hand pod 3 which encloses one end of the chassis. Within that enclosure are carriage-support andxe2x80x94drive mechanics and one end of the printing-medium advance mechanism, as well as a pen-refill station with supplemental ink cartridges.
The printer/plotter also includes a printing-medium roll cover 4, and a receiving bin 5 for lengths or sheets of printing medium on which images have been formed, and which have been ejected from the machine. A bottom brace and storage shelf 6 spans the legs which support the two ends of the case 1.
Just above the print-medium cover 4 is an entry slot 7 for receipt of continuous lengths of printing medium 4. Also included are a lever 8 for control of the gripping of the print medium by the machine.
A front-panel display 11 and controls 12 are mounted in the skin of the right-hand pod 13. That pod encloses the right end of the carriage mechanics and of the medium advance mechanism, and also a printhead cleaning station. Near the bottom of the right-hand pod for readiest access is a standby switch 14.
Within the case 1 and pods 3, 13 a cylindrical platen 41 (FIG. 2)xe2x80x94driven by a motor 42, worm 43 and worm gear 44 under control of signals from a digital electronic processorxe2x80x94rotates to drive sheets or lengths of printing medium 4A in a medium-advance direction. Print medium 4A is thereby drawn out of the print-medium roll cover 4.
Meanwhile a pen-holding carriage assembly 20 carries pens back and forth across the printing medium, along a scanning trackxe2x80x94perpendicular to the medium-advance directionxe2x80x94while the pens eject ink. The medium 4A thus receives inkdrops for formation of a desired image, and is ejected into the print-medium bin 5.
As indicated in the drawing, the image may be a test pattern of numerous color patches or swatches 56, for reading by an optical sensor to generate calibration data. For present purposes, such test patterns are for use in detecting positioning errors.
A small automatic optoelectronic sensor 51 rides with the pens on the carriage and is directed downward to obtain data about pen condition (nozzle firing volume and direction, and interpen alignment). The sensor 51 can readily perform optical measurements 65, 81, 82 (FIG. 7); suitable algorithmic control 82 is well within the skill of the art, and may be guided by the discussions in the present document.
A very finely graduated encoder strip 36 is extended taut along the scanning path of the carriage assembly 20 and read by another, very small automatic optoelectronic sensor 37 to provide position and speed information 37B for the microprocessor. One advantageous location for the encoder strip 36 is immediately behind the pens.
A currently preferred position for the encoder strip 33 (FIG. 3), however, is near the rear of the pen-carriage trayxe2x80x94remote from the space into which a user""s hands are inserted for servicing of the pen refill cartridges. For either position, the sensor 37 is disposed with its optical beam passing through orifices or transparent portions of a scale formed in the strip.
The pen-carriage assembly 20 is driven in reciprocation by a motor 31xe2x80x94along dual support and guide rails 32, 34xe2x80x94through the intermediary of a drive belt 35. The motor 31 is under the control of signals from the digital processor.
Naturally the pen-carriage assembly includes a forward bay structure 22 for pensxe2x80x94preferably at least four pens 23-26 holding ink of four different colors respectively. Most typically the inks are yellow in the left-most pen 23, then cyan 24, magenta 25 and black 26.
Another increasingly common system, however, has inks of different colors that are actually different dilutions for one or more common chromatic colors, in the several pens. Thus different dilutions of black may be in the several pens 23-26. As a practical matter, both plural-chromatic-color and plural-black pens may be in a single printer, either in a common carriage or plural carriages.
Also included in the pen-carriage assembly 20 is a rear tray 21 carrying various electronics. The colorimeter carriage too has a rear tray or extension 53 (FIG. 3), with a step 54 to clear the drive cables 35.
FIGS. 1 through 3 most specifically represent a system such as the Hewlett Packard printer/plotter model xe2x80x9cDesignJet 2000CPxe2x80x9d, which does not include the present invention. These drawings, however, also illustrate certain embodiments of the invention, andxe2x80x94with certain detailed differences mentioned belowxe2x80x94a printer/plotter that includes preferred embodiments of the invention.
(b) Relatively direct PAD-derived bandingxe2x80x94In the images produced by a printer of this type, defects called xe2x80x9cbandingxe2x80x9d appear where unprinted, lightly printed, or double-printed pixel rows occur repetitively. Relatively direct sources of such errors include:
Print-medium-Axis Directionality or Pen-Axis Directionality xe2x80x9cPADxe2x80x9d (FIG. 4), i.e. an angling of inkdrop trajectories particularly near the ends of the nozzle array 12,
variations xcex94PAD in the latter error,
variations xcex94PH in the pen height above the printing medium, and
rotation xcex8z of a printhead about the pen-to-print-medium axis z.
A dominant error source is PAD, which may be seen as analogous to camber. It can be either inboard or outboard.
It operates through the height PH of a pen above the print mediumxe2x80x94including the height variations xcex94PHxe2x80x94to extend or contract the swath length 14 in the pen-length direction on the printing medium, the so-called xe2x80x9cpaper swath lengthxe2x80x9d.
(In this document the term xe2x80x9cheightxe2x80x9d is reserved for actual vertical height of the pen above the medium; hence the dimension of the swath in the long dimension of the medium, though commonly called xe2x80x9cswath heightxe2x80x9d, is here instead termed xe2x80x9cswath lengthxe2x80x9d. Please do not confuse this dimension with the width of the swath, i.e. its dimension transversely across the medium in the carriage-scan axis.)
To reduce visible banding, operating strategies can compensate for such extension or contraction by a matching extension or contraction of the printing-medium-advance stroke. In other words, unprinted or double-printed pixel rows can be avoided by matching the stroke to the actual swath length.
This may be regarded as xe2x80x9ccompensationxe2x80x9d, though in a sense it is the oppositexe2x80x94namely, accommodationxe2x80x94and actually results in image deformation. Both the actual swath length and its matched stroke are different from the nominal swath length that constitutes a basic unit of the image.
Small resulting differences in shapes within an image can sometimes be detected. Accumulated differences due to such deformation can be seen as variation of overall printed image length.
The drawing shows a platen of the roller type, with print medium wrapped around the roller. In this geometry, swath-length extension is aggravated by the fact that the print-medium surface progressively recedes from the nozzle array 12 at all points off-axis relative to the pen-to-medium axis z.
This aggravating effect, in turn, is greatly complicated by any departure of the roller and printing-medium surfaces from nominal conditions, still relatively direct:
radius error xcex94radius from the nominal,
thickness of the printing medium, or difference of that thickness from any assumed nominal thickness,
runout or out-of-round, i.e. departure of the platen from circularity, and
eccentricity of the platen relative to the pen-to-medium axis z, and any difference xcex94eccentricity of that eccentricity from any assumed nominal eccentricity.
The aggravating curved-platen effect itself is still relatively direct, though present only if the system prints on a curved platen 13, such as a roller.
These factors, except for printing-medium thickness variation, are already corrected in factory calibration of the printing-medium advance mechanism. What is particularly problematic is that in a representative product this calibration is inoperative when the system runs in a test-pattern measuring mode.
Hence a test pattern for measuring or accommodating PAD is printed with the advance mechanism calibrated but measured with the mechanism uncalibrated. The resulting measurement of PAD is accordingly imperfect.
The drawing shows the printing medium wrapping around the roller at both ends of the swath (or to left and right as illustrated). In some systems the medium may wrap for example behind the platen but not in frontxe2x80x94still further complicating the overall effect on printed swath length.
In present systems of the greatest accuracy, however, such as precision plotters (as distinguished from systems for printing photograph-like images, or other display graphics), printing is instead on a flat platen. The platen may be formed as a vacuum bed to pull the printing medium into intimate contact.
Hence in such systems the receding-surface contribution is absent. Even if the platen is flat, however, PAD nevertheless directly alters the paper swath length.
(c) Relatively indirect PAD errorsxe2x80x94Furthermore, even if the platen is flat the paper is typically driven by one or more rotary elements such as pinch wheels (not shown), gears 44 (FIG. 2) and worms 43, and so on. Radius error, runout and eccentricities of each of these elements also contribute to error of relative positioning as between the pen and the printing medium.
Depending on their positions in the mechanism relative to the print-medium drive motor, the encoder, and the medium itself (FIG. 5), conceptually these various kinds of devices can introduce mechanical errors of three different types. These types may be effective only as between the encoder and medium, or between the motor considered together with the encoder and the medium, etc.
These relationships require careful consideration in the design of any system for measuring error with an eye to accommodating its effects, as everyone who is skilled in this field can appreciate upon studying the drawing. Once again, in a representative printer/plotter these errors are corrected by factory calibration that is operative and effective when the machine is printing, and even when it is printing a test patternxe2x80x94but not when it is measuring that same test pattern. Consequently the measurement and accommodation of PAD is rendered difficult or inaccurate.
These additional error sources can be regarded as less direct. They can only affect overall position of an entire swath, along the pen-length direction, rather than affecting the length of the pattern.
These sources of error can nevertheless be particularly insidious and troublesome. They are highly variable from one machine to the nextxe2x80x94arising as they do from fabrication tolerances in individual components.
Earlier systems for dealing with PAD problems, and the related ones discussed above, include provisions in the printer/plotter line of the Hewlett Packard Company, particularly for example the previously mentioned model known as the DesignJet 2000CP. It is a tool that is especially optimized for graphic-arts applications.
(d) Deformation of image elements, and alteration of overall image-lengthxe2x80x94As noted above, earlier systems also have produced printouts that are troublesome to users who require overall page or image to be accurately a nominal length - - - or that picture elements within an image be accurately a nominal aspect ratio.
In earlier systems the only available remedy, for such users, is to switch off the PAD-factor accommodation entirely. The result is a Hobson""s choice between banding and image dimensional inaccuracy.
(e) Varying functional requirementsxe2x80x94A limitation of the PAD accommodations practiced heretofore is the capability to investigate and match PAD phenomena with respect to only one characteristic. That characteristic, typically, is overall extension of the effective pen foot-print as defined by measurement using a particular group of end nozzles.
First, for certain kinds of images, characterization of the PAD error even by particular blocks of an established test pattern may not correct the banding artifacts that actually occur. Second, specific pens may exhibit longitudinal PAD profiles that differ greatly from the simple end-camber characteristic suggested by FIG. 4.
Third, pen PAD propertiesxe2x80x94even for given pensxe2x80x94are far from consistent under various nozzle-firing conditions, and the state of the art has not examined the opportunities to select firing properties tailored to a particular requirement. For instance it may be advantageous to use one PAD measurement-and-correction model for one printmode, and another model for a different print-mode.
Similarly, pen directionality is different with different firing frequencies. The stability of pen directionality is greater at some frequencies than others.
Prior PAD measurement techniques have failed to provide means for investigating these conditions for a particular image or type of image.
(f) Conclusionxe2x80x94These limitations have continued to impede achievement of uniformly excellent inkjet printingxe2x80x94at high throughputxe2x80x94on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits. These facets are set forth in the appended claims.